For many years it has been known that peroxyacetic acid in various aqueous forms serves as a powerful antimicrobial agent. See, e.g., F. P. Greenspan, M. A. Johnsen, and P. C. Trexler. Peracetic acid aerosols, Chem. Specialties Mfrs. Assoc. Proc. Ann. Meeting, 42:59-64 (1955) and D. M. Portner and R. K. Hoffman, Sporicidal Effect of Peracetic Acid Vapor, App. Microbiology, 16:1782-1785 (1968). As a result, peroxyacetic acid is widely used in sanitization applications including those involving agricultural storage facilities and processing equipment. The present application discloses a new method of using PAA in these agricultural settings.
Specifically, in California and other citrus production areas, early season navel and mandarin oranges are harvested and treated with ethylene gas for two or more days in humidified “degreening” rooms at 20° C. to accelerate the degradation of residual chlorophyll to enhance the orange color of the fruit rind. Unsurprisingly, these environmental conditions are optimal for the development of green mold (Penicillium digitatum) and blue mold (Penicillium italicum). As a result, post-harvest “bin drenching” with commercial fungicides such as thiabendazole or imazalil is widely used to prevent the development of these pathogens. A predictable consequence of this practice is that fungicide resistant strains of mold develop and contaminate the associated degreening rooms, storage facilities, and packing equipment.
As a result, periodic prophylactic disinfection of empty storage rooms and processing equipment by means of antibacterial fog or mist is a routine occurrence. Formalin solution, which contains formaldehyde as an active ingredient, has long been used for this purpose. Unfortunately, formaldehyde is injurious to the fruit that it comes in contact with and thus may be used only twice yearly when the storage rooms are ordinarily empty. Further, many such storage rooms are never completely empty and are thus unavailable for sanitization by this means. Also, the maximum permissible amount of formaldehyde sanitizer allowed declines progressively as the proximity of inhabited dwellings or schools increases. As a result, other agents including peroxyacetic acid are increasingly used to disinfect such facilities and machinery. But this “two step” approach for separately disinfecting products and facilities is extremely time consuming and expensive. What is needed therefore is a method of using peroxyacetic acid alone to disinfect both the product and the facility in which it is stored and processed. Unfortunately, while various methods of using peroxyacetic acid vapors and/or fogs and/or mists are described in the prior art none are suitable for this purpose.
For example, U.S. Pat. Pub. No. 2010/0196197 discusses injecting peroxyacetic acid in a concentration ranging from 4,000 PPM to 10,000 PPM into a “heated gas stream” comprised of “sterile air” and/or other gases heated to a temperature “above about 250° C.” The resulting heated vapor is then applied at a temperature ranging between, “about 80° C. and about 120° C.” to, “metal, plastics, polymers, and elastomers” for a period of time ranging between, “15 and about 40 minutes.” More recently, U.S. Pat. Pub. No. 2012/0189494 further refines this method for use in the context of heat sensitive polyethylene terephthalate (PET) bottles by disclosing a method in which the application temperature is lowered considerably to a range between, “about 57° C. and about 75° C.” and the amount of time the vapor is actually applied ranges as low as, “5 seconds.”
While these approaches may be useful for sterilizing inert, solid surfaces they are unsuitable for use on delicate organic surfaces such as those found on agricultural products. This is because the surfaces found on agricultural products are relatively rough compared to most inert, solid surfaces and the treatment times necessary to destroy pathogens must be greatly increased. Unfortunately, because of the relatively high application temperatures, increasing the treatment time has the unwanted side effect of damaging the agricultural product. Further, the methods discussed above require a source of sterile air or a source of some other sterile gas and as such both are unsuitable for use in the field where sterile compressed air or other gases may be unavailable.
Clearly, a method of generating and applying peroxyacetic acid vapor at temperatures and under conditions suitable for its application to non-inert surfaces such as those found on agricultural products would be advantageous as a means of sanitizing fruits, tubers, and vegetables in situ in the storage facility while simultaneously sanitizing the storage facility itself. Also, a method of generating peroxyacetic acid vapor using atmospheric air would have similar utility since many of the agricultural and other facilities where the process might be used lack ready access to sterile compressed air or other sterile gasses.